Mem. Fac. Educ. Kumamoto. Univ. Nat. Sci., No. 34, 5—17, 1985 Larval Development of the Hermit Crab Diogenes nitidimanus TERAO, 1913 (Crustacea : Anomura : Diogenidae) Reared in the Laboratory* Keiji BABA and Yasushi FUKUDA (Received 23 May 1985) The complete larval development of the hermit crab Diogenes nitidimanus from hatching to first crab stage was obtained by laboratory rearing. Four zoeal and one megalopal stages are described and illustrated. The pleopod buds appeared in the third zoea, the posterior lobe of the scaphognathite appeared in the second zoea, and the antenna in the megalopa showing a remark- able differentiation toward suspension feeding, all unique to D. nitidimanus, are discussed. Introduction The hermit crab genus Diogenes represented by about 46 species are sublittoral inhabitants, many species of which are known from the Indo West-Pacific. In spite of the recent increase of species descriptions the systematics of the genus requires revision from a view point of functional morphology relating especially to the feeding habits, which will be published elsewhere. Knowledge of the larval development of this genus is very limited, restricted to thorough descriptions of D. pugilator by MACDONALD, PIKE and WILLIAMSON (1957), D. bicristimanus by SAROJINI and NAGABUSHANAM (1968), D. avarus by SANKOLLI and SHENOY (1975) and D. diogenes by NAYAK and KAKATI (1977). Planktonic material has also been described earlier for D. pugilator (MENON, 1937). Diogenes nitidimanus is a typical suspension feeder and is one of the common shore hermit crabs in Kyushu, Japan ; when the tide recedes the animals are sometimes found buried partly on muddy sand substrates at low tide levels, but usually they are not exposed to the air. This paper describes the complete larval development of D. nitidimanus obtained by rearing in the laboratory. Herein discussed are some of the larval characters uniquely displayed by D. nitidimanus, which seem at variance with previous definitions especially of the infraorder Anomura and the develop- mental sequence of scaphognathites, or, which may expand the definition of Diogenes. Attention is also accorded to the functional morphology of the antenna in the postlarval stages, the flagellum with a row of long plumose setae being associated with net-casting or suspension feeding. Methods and Materials The ovigerous female was collected on 25 June 1979 from the intertidal zone in Amakusa, Kumamoto, the west coast of Kyushu, Japan. The first zoeas were obtained on 29 June 1979 and reared under laboratory temperatures of 26.0 - 29.0°C. The rearing techniques follow FUKUDA * This paper was read at the 21th annual meeting of the Japanese Society of Systematic Zoology held at the National Science Museum, Tokyo, 3 April 1985. (5) 6 Keiji BABA and Yasushi FUKUDA (1982). Length of carapace was measured at midline between the tip of the rostrum to the posterior end of the carapace in five specimens. Setae of appendages are formulated toward the distal end, unless otherwise mentioned. Results Diogenes nitidimanus passed through four zoeal and one megalopal stages before reaching the first crab stage. The number of days required to complete the larval development in the laboratory (filtered sea water, Artemia nauplii fed) was 20 days. The major characteristics are described below. FIRST ZOEA Carapace length.— 1.14mm (average). Duration of stage. — Four days. Carapace (Fig. la). — About as long as abdomen, inclusive of rostral spine ; smooth, dorsoante- riorly concave moderately, anterolateral^ with tiny but distinct spine, posterolateral^ rounded. Rostrum horizontal, only slightly shorter than remaining carapace when measured from postorbital margin, overreaching antennal scale but not its distal marginal setae. Eyes sessile. Antennule (Fig. lb). — Elongate rod, provided terminally with 4 aesthetascs and 2 plumose, shorter setae, subterminally with pronounced plumose seta. Antenna (Fig. lc). — Endopod not divided from peduncle, provided terminally with 2 well- developed plumose setae, subterminally with very short one. Peduncle with laterally serrate, spiniform process on ventral terminal margins. Antennal scale fully 1.5 times as long as endopod, provided with outer terminal spine, and fringed with 11 plumose setae on inner and distal margins ; outermost seta directly inside terminal spine rather short and somewhat ventral in position. Mandible (Fig. Id). — Almost symmetrical. Incisor process acute, with serrate accessory process on dorsoproximal portion. Molar process with submarginal denticles. Maxillule (Fig. le). — Palp 2-segmented, distal segment very short, provided with 2 simple terminal setae. Coxal endite with 6 plumose, bristle-like setae. Basal endite with 2 strong spines, each with row of marginal denticles. Maxilla (Fig. If). — Palp unsegmented, with 2 terminal setae. Coxal endite also bilobed, with 7 setae on proximal lobe, 4 on distal lobe (formulated as 7, 4). Basal endite also bilobed, proximal lobe with 5 setae, distal lobe 3 setae (5, 3). Scaphognathite lacking posterior lobe, anterior lobe with 5 plumose setae. First maxilliped (Fig. lg). — Setation of basis, 1, 3, 2 ; endopod 5-segmented, with setal formula of 3-2-1-2-5. Exopod constricted at midlength, with 4 natatory setae. Second maxilliped (Fig. lh). — Setation of basis, 1,1; endopod 4-segmented, its setation, 2-2-3-4. Exopod much like that of preceding appendage. Third maxilliped (Fig. li). — Uniramous, elongate simple bud. Abdomen. — Sixth segment not separated from telson. Fourth segment with middorsal spine posteriorly. Fifth segment with lateral and middorsal spines posteriorly. Telson (Fig. lj). — Triangular, posterior median sinus distinct. Telson processes, 7 + 7; first process spiniform, not articulated at base, second process hair-like and plumose ; remainder seta-like but pronounced. Fig. 1. Diogenes nitidimanus TERAO, first zoea: a, lateral view; b, antennule; c, antenna ; d, mandible ; e, maxillule ; f, maxilla ; g, first maxilliped ; h, second maxilliped ; i, third maxilliped ; j, telson. Scale for a, 0.2 mm ; for b-j, 0.1 mm. 8 Keiji BABA and Yasushi FUKUDA Fig. 2. Diogenes nitidimanus TERAO, second zoea : a, lateral view ; b, antennule; c, maxillule; d, maxilla ; e, third maxilliped. Scale for a, 0.2 mm ; for b-e, 0.1 mm. SECOND ZOEA Carapace length.— 1.24 mm (average). Duration of stage. — Four days. Carapace (Fig. 2a). — Little larger than in previous stage, retaining anterolateral spine. Rostral spine about as long as remaining carapace. Eyes movable. Antennule (Fig. 2b). — Terminally with 3 aesthetascs, 3 simple and 1 plumose setae; pronounced subterminal plumose seta accompanying few fine, plumose setae directly dorsal to it. Antenna. — Almost as in first zoea, endopod articulate basally. Mandible. — Marginal denticles of molar process more numerous. Maxillule (Fig. 2c). — Short plumose seta newly appeared on proximal segment of palp. Setation of coxal endite unchanged ; basal endite with 4 strong marginal spines and 1 tiny seta slightly ventral to margin. Larval Development of Diogenes 9 Maxilla (Fig. 2d). — Setae on coxal and basal endites and on palp as in first zoea. Scaphognathite with setaless posterior lobe ; marginal setae of anterior lobe increased to 8. First maxilliped. — Endopod setae, 4-3-2-2-5 ; natatory setae increased to 6 ; otherwise unchanged. Second maxilliped. — Setation of basis unchanged ; midventral seta added to penultimate segment of endopod ; 6 natatory setae. Third maxilliped (Fig. 2e). — Biramous; endopod simply rod-like; exopod with 5 natatory setae. Abdomen and telson. — Unchanged morphologically except size. THIRD ZOEA Carapace length.— 1.42mm (average). Duration of stage. — Five days. Carapace. — Nearly as in preceding stage. Antennule (Fig. 3a). — Terminally with group of 3 aesthetascs and 4 setae. Inner ramus short, ending in pronounced plumose seta, flanked by pair of long plumose setae, and accompanying 3 fine setae directly outside to it. Antenna (Fig. 3b). — Endopod with rudimentary terminal seta. Antennal scale relatively narr- owed, with 12 marginal setae. Terminal process of basis reduced in size. Mandible. — Nearly as in second zoea. Maxillule. — Mostly as in second zoea, but coxal endite setae increased to 7. 10 Keiji BABA and Yasushi FUKUDA Fig. 4. Diogenes nitidimanus TERAO, fourth zoea : a, lateral view; b, antennule; c, antenna ; d, mandible ; e, maxillule ; f, maxilla ; g, first maxilliped ; h, second maxilliped ; i, third maxilliped ; j, right cheliped ; k, left cheliped ; 1, telson. Scale for a, 0.5 mm ; for b-1, 0.1 mm. Larval Development of Diogenes 11 Maxilla. — Posterior lobe of scaphognathite more elongate than in second zoea. Marginal setae of anterior lobe increased to 13. Setations of palp and endites unchanged. First and second maxillipeds. — Nearly as in preceding stage. Third maxilliped. — Endopod arising from proximal portion of basis, not overreaching end of basis; 6 natatory setae. Pereopods. — Only first pereopod appeared as bud, biramous, suggesting chela. Abdomen. — Buds of pleopods on second through fourth abdominal segments. Sixth segment separated from telson. Uropod appeared, rami unarticulated, inner ramus (endopod) bud-like, setaless; outer ramus (exopod) with elongate terminal spine and fringed with 9 plumose setae on inner margin. Telson (Fig. 3c). — Posterior margin feebly concave, without distinct sinus, provided with 7 + 7 processes. FOURTH ZOEA Carapace length.— 1.93mm (average). Duration of stage. — Three days. Carapace (Fig. 4a).